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Ascending Spinal Cord Pathways Neuroanatomy > Spinal Cord & Sensory Projections > Spinal Cord & Sensory Projections Ascending Spinal Cord Pathways Neuroanatomy > Spinal Cord & Sensory Projections > Spinal Cord & Sensory Projections ASCENDING SPINAL CORD PATHWAYS SUMMARY ANATOMY Posterior Column Pathway - Fiber size: large - Function: carry proprioceptive sensation (sense of position) – vibration, two-point discrimination, and joint position information - Course: ascends the spinal cord ipsilateral to their side of the origin - Cuneate fasciculus carries sensory information from the upper body (from T6 and above) - Gracile fasciculus carries sensory information form the lower body (from T6 and below) Clinical Correlation: Subacute Combined Degeneration Anterolateral system (aka spinothalamic tract) • Fiber size: small • Function: carry pain, temperature, and itch sensation • Course: ascends the spinal cord contralateral to their side of origin • Inner fibers: arms • Outer fibers: leg - Generally, the lateral regions of the anterolateral system subserve pain and temperature and the anterior regions subserve tactile and pressure sensation. Spinocerebellar pathways - Fiber size: large 1 / 5 - Function: carry joint proprioception to the cerebellum for coordination - Posterior and anterior sections PATHWAYS Dorsal Root Ganglia • Cell bodies lie within the dorsal root ganglia. • Each cell body contains a short axon. Anterolateral system - Passes peripherally and centrally to join the motor root toward the spinal cord - Lissauer's tract: entry zone for fibers along the dorsal edge of the dorsal horn - Ventral commissure: fibers cross midline between the anterior horns of the spinal cord Posterior column pathway - Directly ascends the spinal cord within the ipsilateral posterior column - Posterior median septum separates the bilateral sides of the posterior column - The posterior intermediate septum separates ipsilateral fasciculi CLINICAL CORRELATION Syringomyelia • A cervical syringomyelia occurs when a fluid-filled cavity (a syrinx) expands outward and encroaches upon the anterolateral system, which affects the inner (arm) fibers prior to the outer (leg) fibers as well as affecting the crossing fibers at that level. FULL TEXT OVERVIEW • Here, we will draw the ascending spinal cord pathways, which carry sensory information from the body. Sensory afferents from the face travel via the trigeminal sensory system, which is discussed elsewhere. • First, start a table for the key ascending pathways. 2 / 5 Denote that they are the: • Posterior column pathway • Anterolateral system (often referred to simply as the spinothalamic tract) • Spinocerebellar pathways. Axial Cross-Section • Now, let's draw an axial cross-section of these pathways. • Draw an outline of the spinal cord. • Next, draw the butterfly-wing shaped gray matter horns. • Then, draw the bilateral posterior columns. Innervation Table Begin a table for the sensory modalities of the Ascending Pathways. • Label the header: Pathway, Fiber Size, Function, and Course; we'll see how fiber size and function are inter-related. Indicate that the posterior column pathway comprises large sensory fibers that carry proprioceptive sensation (our sense of position) — vibration, two-point discrimination, and joint position information. POSTERIOR COLUMN PATHWAY • Next, indicate that posterior column fibers ascend the spinal cord ipsilateral to their side of origin – they do NOT cross to the opposite side. • Next, indicate that a division occurs at thoracic level 6 (T6). • Show that the cuneate fasciculus carries sensory information from the upper body (from T6 and above, eg the arms) Indicate that the gracile fasciculus carries sensory information from the lower body (from below T6, eg, the legs). • As a mnemonic, for the gracile fasciculus, draw a ballerina – ballerinas need graceful legs, which helps associate gracile with legs. ANTEROLATERAL SYSTEM (THE SPINOTHALAMIC TRACT) • Now, show that the anterolateral system (the spinothalamic tract) lies just outside of the anterior gray matter horn (in the anterior/lateral region of the white matter of the spinal cord). • Indicate in our table that the anterolateral system comprises small fiber sensory pathways that carry pain, temperature, and itch sensory information; it carries what's bothersome or painful. • Although the anterolateral system most notably includes the spinothalamic tract (and thus is sometimes considered synonymous to the spinothalamic tract), it doesn't just synapse in the thalamus but also in the hypothalamus and brainstem, which is why we refer to it as the anterolateral system. 3 / 5 • Next, indicate that anterolateral system fibers ascend the spinal cord contralateral to their side of origin – they cross to the opposite side (unlike the posterior column pathway). • Now, show that its somatotopic organization is such that the arm fibers lie on the inside and the leg fibers lie on the outside. Cervical syringomyelia • As a mneomonic, let's learn the clinical correlation of a cervical syringomyelia. • Draw an outline of the cervical spinal cord. • Then, draw the gray matter horns. • Next, demarcate the anterolateral system in the anterolateral spinal cord. • Then, show its fibers cross the ventral commissure. • Now, show a fluid-filled cavity (a syrinx) expand outward and encroach upon the anterolateral system to show that it affects the inner (arm) fibers prior to the outer (leg) fibers, and it also affects the crossing fibers at that level. • To understanding the clinical exam findings of a cervical syringomyelia, draw a person with a suspended-sensory level wherein small fiber sensation is lost in a cape-like distribution in the arms and upper trunk, but sensation is preserved in the legs. Organization of the anterolateral system • Next, let's transition to a separate organizational principle of the anterolateral system. As a division/function correlate: • Generally, the lateral regions of the anterolateral system subserve pain and temperature and the anterior regions subserve tactile and pressure sensation. SPINOCEREBELLAR TRACTS • Lastly, along lateral wall of the spinal cord, draw the spinocerebellar tracts. Indicate that they divide into: • Posterior spinocerebellar tracts. • Anterior spinocerebellar tracs. • Write that the spinocerebellar tracts comprise large sensory fibers that carry joint proprioception to the cerebellum for coordination. PATHWAYS 4 / 5 Now, that we've learned the anatomy of these systems, let's draw out their pathways. • First, show that the cell bodies for spinal sensory fibers lie within dorsal root ganglia. • Show that they contain a single short axon that emanates from the cell body. • Indicate that it passes peripherally (to join the motor root). • And centrally (toward the spinal cord). • Thus, the spinal sensory axons are pseudo-unipolar. Anterolateral System • Now draw Lissauer's tract along the dorsal edge of the dorsal horn, which is the entry zone for the anterolateral system fibers. • Next, label the ventral commissure between the anterior horns of the spinal cord, which is where the anterolateral system fibers cross midline. • With those regions in place, show that the central processes of the anterolateral system pass through Lissauer's tract and synapse within the ipsilateral dorsal horn. • Then, show that they cross within the ventral commissure to ascend the spinal cord within the anterolateral system (contralateral to their side of origin). Posterior Column Pathway • Now, we're ready for the posterior column pathway. • Indicate that the posterior column pathway fibers directly ascend the spinal cord within the ipsilateral posterior column, thus, they do not make a synapsis within the spinal cord. • Finally, let's introduce some important posterior column demarcation lines. • Show that the posterior median septum separates the bilateral sides of the posterior columns. • Then, show that the posterior intermediate septum separates ipsilateral fasciculi. Powered by TCPDF (www.tcpdf.org) 5 / 5.
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